Acoustic wave device

ABSTRACT

An acoustic wave device includes: a piezoelectric film made of an aluminum nitride film containing a divalent element and a tetravalent element, or a divalent element and a pentavalent element; and an electrode that excites an acoustic wave propagating through the piezoelectric film.

The present reissue application is a reissue application of U.S.application Ser. No. 13/787,497, filed Mar. 6, 2013, now U.S. Pat. No.9,087,979.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application Nos. 2012-058441 filed on Mar. 15,2012 and 2012-250535 filed on Nov. 14, 2012, the entire contents ofwhich are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to acoustic wavedevices.

BACKGROUND

Diffusion of wireless communication devices including mobile phones hasencouraged development of filters formed by combining acoustic wavedevices using a surface acoustic wave (SAW) or bulk acoustic wave (BAW).The filter using a SAW or BAW has a small outer shape and a high Qcompared to a dielectric filter, and thus is suitable for ahigh-frequency component in a wireless communication device such as amobile phone required to be small in size and have a steep skirtcharacteristic. Moreover, there has been suggested an acoustic wavedevice using a Lamb wave as a developed device of the acoustic wavedevice using a SAW or BAW.

In recent years, filters are required to have high performance. Forexample, a bandwidth of a filter characteristic is required to bewidened. The bandwidth of the filter characteristic can be widened byincreasing an electromechanical coupling coefficient of an acoustic wavedevice used in the filter. Use of a piezoelectric film with a highelectromechanical coupling coefficient can increase theelectromechanical coupling coefficient of the acoustic wave device.

When an aluminum nitride film is used as the piezoelectric film, theelectromechanical coupling coefficient of the acoustic wave device canbe improved by controlling a c-axis orientation of the aluminum nitridefilm as disclosed in Rajan S. Naik, and 10 others, “Measurements of theBulk, C-Axis Electromechanical Coupling Constant as a Function of AlNFilm Quality”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS ANDFREQUENCY CONTROL, 2000, vol. 47, p. 292-296 (Non-Patent Document 1),for example. For example, the electromechanical coupling coefficient ofthe acoustic wave device can be improved by using an aluminum nitridefilm containing an alkali earth metal and/or a rare-earth metal for thepiezoelectric film as disclosed in Japanese Patent ApplicationPublication No. 2002-344279 (Patent Document 1). Moreover, piezoelectricresponse of the acoustic wave device can be improved by using analuminum nitride film containing scandium at a content rate in apredetermined range for the piezoelectric film as disclosed in JapanesePatent Application Publication No. 2009-10926 (Patent Document 2).

However, the art disclosed in Non-Patent Document 1 aims to improve theelectromechanical coupling coefficient of the aluminum nitride film, andthus fails to obtain an electromechanical coupling coefficient higherthan that obtained from a material characteristic of the aluminumnitride film. In addition, the art disclosed in Patent Document 1 aimsto improve the electromechanical coupling coefficient by increasing abond concentration of a grain boundary between c-axis oriented aluminumnitride particles, and thus fails to obtain an electromechanicalcoupling coefficient higher than that obtained from a materialcharacteristic of the aluminum nitride film.

The acoustic wave device grows in size and thus increases cost as aresonance frequency decreases.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anacoustic wave device including: a piezoelectric film made of an aluminumnitride film containing a divalent element and a tetravalent element, ora divalent element and a pentavalent element; and an electrode thatexcites an acoustic wave propagating through the piezoelectric film.

According to another aspect of the present invention, there is providedan acoustic wave device including: a piezoelectric film made of analuminum nitride film containing an element that achieves at least oneof an increase in a permittivity and a decrease in an acoustic velocity;and an electrode that excites an acoustic wave propagating through thepiezoelectric film, wherein a resonance frequency is less than or equalto 1.5 GHz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structure of aluminum nitride used for simulations;

FIG. 2A is a top view of an acoustic wave device in accordance with afirst embodiment, FIG. 2B is a cross-sectional view taken along line A-Ain FIG. 2A, and FIG. 2C is a cross-sectional view taken along line B-Bin FIG. 2A,

FIG. 3A through FIG. 3H are cross-sectional views for explaining afabrication method of the acoustic wave device of the first embodiment;

FIG. 4A illustrates simulation results of a resonance characteristic ofa first FBAR, and FIG. 4B illustrates simulation results of a resonancecharacteristic of a second FBAR;

FIG. 5 illustrates simulation results of a band structure of first dopedaluminum nitride;

FIG. 6 illustrates simulation results of a band structure of seconddoped aluminum nitride;

FIG. 7 illustrates simulation results of a band structure of third dopedaluminum nitride;

FIG. 8 illustrates a relationship between a piezoelectric constant e₃₃and an electromechanical coupling coefficient k²;

FIG. 9 illustrates a relationship between a ratio (c/a) of a latticeconstant in a c-axis direction to a lattice constant in an a-axisdirection and an electromechanical coupling coefficient k²;

FIG. 10A illustrates a dependence of an electromechanical couplingcoefficient k² on substitutional concentrations when magnesium is usedas a divalent element and hafnium is used as a tetravalent element, andFIG. 10B illustrates a dependence of an electromechanical couplingcoefficient k² on substitutional concentrations when magnesium is usedas a divalent element and titanium is used as a tetravalent element;

FIG. 11 is a cross-sectional view of an acoustic wave device inaccordance with a first variation of the first embodiment;

FIG. 12A illustrates simulation results of a resonance characteristic ofa third FBAR, and FIG. 12B illustrates simulation results of a resonancecharacteristic of a fourth FBAR;

FIG. 13 illustrates simulation results of a resonance characteristic ofa fifth FBAR;

FIG. 14 illustrates simulation results of a band structure of fourthdoped aluminum nitride;

FIG. 15 illustrates simulation results of a band structure of fifthdoped aluminum nitride;

FIG. 16 illustrates a relationship between a piezoelectric constant e₃₃and an electromechanical coupling coefficient k²;

FIG. 17 illustrates a relationship between a ratio (c/a) of a latticeconstant in a c-axis direction to a lattice constant in an a-axisdirection and an electromechanical coupling coefficient k²;

FIG. 18 illustrates a dependence of an electromechanical couplingcoefficient k² on substitutional concentrations when magnesium is usedas a divalent element and tantalum is used as a pentavalent element;

FIG. 19 illustrates simulation results of a resonance characteristic ofa sixth FBAR;

FIG. 20A illustrates a relationship between a total of substitutionalconcentrations of Mg and Zr and a normalized piezoelectric constant, andFIG. 20B is a diagram that extracts data of which a ratio ofsubstitutional concentrations of Mg to Zr is around 1:1 from FIG. 20A;

FIG. 21A illustrates a relationship between a ratio of a substitutionalconcentration of Zr to a total of substitutional concentrations of Mgand Zr and a normalized piezoelectric constant; and FIG. 21B is adiagram that extracts data of which a total of substitutionalconcentrations of Mg and Zr is greater than or equal to 3 atomic % andless than or equal to 10 atomic % from FIG. 21A;

FIG. 22A and FIG. 22B illustrate relationships between a total ofsubstitutional concentrations of a divalent element and a tetravalentelement and a normalized piezoelectric constant;

FIG. 23 illustrates a relationship between a total of substitutionalconcentrations of Mg and Zr and a c/a ratio;

FIG. 24A illustrates a relationship between a resonance frequency and anormalized film thickness of a resonance portion of an FBAR of a thirdcomparative example, and FIG. 24B illustrates a relationship between aresonance frequency and a normalized area of the resonance portion;

FIG. 25A illustrates a relationship between a substitutionalconcentration of Sc and a permittivity ∈₃₃, and FIG. 25B illustrates arelationship between a substitutional concentration of Sc and anacoustic velocity V;

FIG. 26A illustrates a relationship between a resonance frequency and anormalized film thickness of a resonance portion of an FBAR of a fourthembodiment, and FIG. 26B illustrates a relationship between a resonancefrequency and a normalized area of the resonance portion;

FIG. 27A is a cross-sectional view of an acoustic wave device inaccordance with a first variation of the fourth embodiment; and FIG. 27Bis a cross-sectional view of an acoustic wave device in accordance witha second variation of the fourth embodiment;

FIG. 28A illustrates relationships between a resonance frequency and anormalized film thickness of a resonance portion in the first variationand the second variation of the fourth embodiment; and FIG. 28Billustrates a relationship between a resonance frequency and anormalized area of the resonance portion;

FIG. 29A is a cross-sectional view of an FBAR in accordance with a firstvariation of the embodiments, FIG. 29B is a cross-sectional view of anFBAR in accordance with a second variation of the embodiments, and FIG.29C is a cross-sectional view of an SMR;

FIG. 30 is a cross-sectional view of a CRF;

FIG. 31A is a top view of a surface acoustic wave device, and FIG. 31Bis a cross-sectional view taken along line A-A in FIG. 31A, FIG. 31C isa cross-sectional view of a Love wave device, and FIG. 31D is across-sectional view of a boundary acoustic wave device; and

FIG. 32 is a cross-sectional view of a Lamb wave device.

DETAILED DESCRIPTION

Hereinafter, a description will be given of embodiments of the presentinvention with reference to the attached drawings.

First Embodiment

A description will now be given of a simulation for aluminum nitride(AlN) conducted by the inventors. The simulation was conducted with amethod called a first principle calculation. Methods of calculating anelectronic state without using fitting parameters or the like arecollectively referred to as the first principle calculation, which cancalculate the electronic state by using only atomic numbers andcoordinates of atoms constituting a unit lattice or molecule. FIG. 1illustrates a structure of AlN used for the simulation. As illustratedin FIG. 1, used for the simulation is AlN with a wurtzite-type crystalstructure that is a supercell containing sixteen aluminum atoms 10 andsixteen nitrogen atoms 12 obtained by doubling a unit lattice containingtwo aluminum atoms 10 and two nitrogen atoms 12 in a-axis, b-axis, andc-axis directions. The first principle calculation is performed to theAlN with the wurtzite-type crystal structure by moving an atomiccoordinate, a cell volume, and a cell shape simultaneously, and theelectronic state of the AlN in a stable structure is calculated.

Table 1 presents a lattice constant in the a-axis direction, a latticeconstant in the c-axis direction, and a ratio (c/a) of the latticeconstant in the c-axis direction to the lattice constant in the a-axisdirection calculated from the electronic state of the AlN in the stablestructure obtained by the first principle calculation. Table 1 alsopresents experimental values obtained by actually forming an AlN film bysputtering and measuring the AlN film by X-ray diffraction.

TABLE 1 Lattice constant in Lattice constant in a-axis direction [Å]c-axis direction [Å] c/a Calculated value 3.11 4.98 1.60 Experimentalvalue 3.11 4.98 1.60

As presented in Table 1, both the calculation value and the experimentalvalue are 3.11 [Å] with respect to the lattice constant in the a-axisdirection, 4.98 [Å] with respect to the lattice constant in the c-axisdirection, and 1.60 with respect to the c/a ratio. This resultdemonstrates that the above-described simulation using the firstprinciple calculation is valid.

A description will now be given of a simulation for doped AlN doped withan element other than aluminum (Al) and nitrogen (N). Hereinafter, AlNthat is not doped with an element other than Al and N is referred to asnon-doped AlN. The simulation is performed to doped AlN with a crystalstructure formed by substituting a divalent element in one of thealuminum atoms 10 and substituting a tetravalent element in another oneof the aluminum atoms 10 in non-doped AlN with the wurtzite-type crystalstructure described in FIG. 1. That is to say, the simulation isperformed to the doped AlN with the wurtzite-type crystal structurecontaining fourteen aluminum atoms, one divalent element, onetetravalent element, and sixteen nitrogen atoms formed by substituting adivalent element and a tetravalent element in a part of aluminum sites.Here, referred to as a substitutional concentration is an atomicconcentration of a substitution element when a total of the number ofaluminum atoms and the number of atoms of the substitution elementdefines 100 atomic %. Thus, the divalent element and the tetravalentelement contained in the doped AlN for the simulation havesubstitutional concentrations of 6.25 atomic %. Calcium (Ca), magnesium(Mg), strontium (Sr), or zinc (Zn) is used as the divalent element, andtitanium (Ti), zirconium (Zr), or hafnium (Hf) is used as thetetravalent element.

As is the case with the non-doped AlN, the first principle calculationcan calculate an electronic state of the doped AlN in the stablestructure, and the calculated electronic state allows a lattice constantin the a-axis direction, a lattice constant in the c-axis direction, anda c/a ratio to be calculated. The first principle calculation can alsocalculate piezoelectric constants, elastic constants, and permittivitiesof the non-doped AlN and the doped AlN from minor change of total energycaused by a small strain forcibly applied to the crystal lattices of thenon-doped AlN and the doped AlN in the stable structure. A relationalexpression (Expression 1) holds true among a piezoelectric constant e₃₃,an elastic constant C₃₃, and a permittivity ∈₃₃ in the c-axis directionand an electromechanical coupling coefficient k² (hereinafter, referredto as k²). Therefore, electromechanical coupling coefficients k² of thenon-doped AlN and the doped AlN can be calculated by calculatingpiezoelectric constants e₃₃, elastic constants C₃₃, and permittivities∈₃₃ of the non-doped AlN and the doped AlN respectively.

$\begin{matrix}{k^{2} = \frac{e_{33}^{2}}{ɛ_{33} \times C_{33}}} & \lbrack {{Expression}\mspace{14mu} 1} \rbrack\end{matrix}$

Table 2 presents calculated piezoelectric constants e₃₃ and k²calculated from Expression 1 of the non-doped AlN and the doped AlN. Aspresented in Table 2, the obtained results demonstrate that the dopedAlN doped with a divalent element and a tetravalent element (Case 1through Case 10) have piezoelectric constants e₃₃ and electromechanicalcoupling coefficients k² greater than those of the non-doped AlN(Non-doped AlN in Table 2). A combination of the divalent element andthe tetravalent element may be Ca—Ti, Ca—Zr, Ca—Hf, Mg—Ti, Mg—Zr, Mg—Hf,Sr—Hf, Zn—Ti, Zn—Zr, or Zn—Hf as presented in Table 2, and may be othercombinations.

TABLE 2 Electro- mechanical Piezoelectric coupling Combi- DivalentTetravalent constant e₃₃ coefficient nation element element [C/m²] k²[%] Case 1 Ca Ti 1.77 9.68 Case 2 Ca Zr 1.85 10.3 Case 3 Ca Hf 2.17 14.2Case 4 Mg Ti 2.09 12.9 Case 5 Mg Zr 2.13 13.5 Case 6 Mg Hf 2.46 17.6Case 7 Sr Hf 1.96 11.3 Case 8 Zn Ti 2.08 12.5 Case 9 Zn Zr 2.01 12.4Case 10 Zn Hf 2.32 11.1 Non-doped — — 1.55 7.12 AlN

As presented above, the inventors have newly found that doped AlNcontaining a divalent element and a tetravalent element has anelectromechanical coupling coefficient k² greater than that of non-dopedAlN. Thus, a description will now be given of a first embodiment capableof obtaining an acoustic wave device having a high electromechanicalcoupling coefficient k² based on the above knowledge.

FIG. 2A is a top view of an acoustic wave device in accordance with thefirst embodiment, FIG. 2B is a cross-sectional view take along line A-Ain FIG. 2A, and FIG. 2C is a cross-sectional view taken along line B-Bin FIG. 2A. The first embodiment describes an FBAR (Film Bulk AcousticResonator) that is one of piezoelectric thin film resonators. Asillustrated in FIG. 2A through FIG. 2C, an FBAR 20 includes a substrate22, a lower electrode 24, a piezoelectric film 26, and an upperelectrode 28.

The substrate 22 may be an insulative substrate such as a silicon (Si)substrate, a glass substrate, a gallium arsenide (GaAs) substrate, or aceramic substrate. The lower electrode 24 is located on the substrate22. The lower electrode 24 may be a metal film including at least one ofaluminum (Al), copper (Cu), chrome (Cr), molybdenum (Mo), tungsten (W),tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), and iridium(Ir) for example. The lower electrode 24 may have a single layerstructure or a multilayer structure.

The piezoelectric film 26 is located on the substrate 22 and the lowerelectrode 24. The piezoelectric film 26 is an aluminum nitride (AlN)film containing a divalent element and a tetravalent element, and has acrystal structure with a c-axis orientation that has a c-axis as a mainaxis. The divalent element and the tetravalent element are substitutedin aluminum sites of the aluminum nitride film. The upper electrode 28is located on the piezoelectric film 26 so as to have a region facingthe lower electrode 24. A resonance portion 30 is a region where thelower electrode 24 and the upper electrode 28 face each other across thepiezoelectric film 26. The upper electrode 28 may be a metal filmincluding at least one of Al, Cu, Cr, Mo, W, Ta, Pt, Ru, Rh, and Irdescribed for the lower electrode 24. The upper electrode may have asingle layer structure or a multilayer structure.

A dome-shaped air-space 32 is located between the substrate 22 and thelower electrode 24 in the resonance portion 30. The dome-shapedair-space 32 has a height that becomes higher as it becomes closer to acenter of the air-space 32. An introduction path 34 formed byintroduction of etchant for forming the air-space 32 is located belowthe lower electrode 24. The piezoelectric film 26 or the like does notcover a vicinity of a tip of the introduction path 34, and the tip ofthe introduction path 34 forms a hole portion 36. The hole portion 36 isan inlet for introducing etchant to form the air-space 32. An aperture38 is formed in the piezoelectric film 26 to provide an electricalconnection with the lower electrode 24.

When a high frequency electrical signal is applied between the lowerelectrode 24 and the upper electrode 28, an acoustic wave excited by theinverse piezoelectric effect or an acoustic wave caused by a strain dueto the piezoelectric effect is generated in the piezoelectric film 26sandwiched by the lower electrode 24 and the upper electrode 28. Theabove-described acoustic wave is fully reflected at surfaces exposed toair of the lower electrode 24 and the upper electrode 28, and thusbecomes a bulk acoustic wave having a main displacement in a thicknessdirection. That is to say, the lower electrode 24 and the upperelectrode 28 function as electrodes exciting an acoustic wavepropagating through the piezoelectric film 26.

A description will now be given of a fabrication method of the acousticwave device of the first embodiment with reference to FIG. 3A throughFIG. 3H. FIG. 3A through FIG. 3D are cross-sectional views correspondingto a cross-section taken along line A-A in FIG. 2A, and FIG. 3E throughFIG. 3H are cross-sectional views corresponding to a cross-section takenalong line B-B in FIG. 2A.

As illustrated in FIG. 3A and FIG. 3E, a sacrifice layer 39 is formed onthe substrate 22 by sputtering or evaporation. The sacrifice layer 39 ismade of magnesium oxide (MgO) for example, and is formed in at least aregion in which the air-space 32 is to be formed. The sacrifice layer 39may have a film thickness of 20 nm for example. A metal film is thenformed on the substrate 22 and the sacrifice layer 39 by sputtering inan argon (Ar) gas atmosphere for example. The metal film is selectedfrom at least one of Al, Cu, Cr, Mo, W, Ta, Pt, Ru, Rh, and Ir asdescribed previously. Then, the metal film is shaped into a desiredshape by exposure and etching to form the lower electrode 24. At thispoint, a part of the lower electrode 24 has a shape that covers thesacrifice layer 39.

As illustrated in FIG. 3B and FIG. 3F, the piezoelectric film 26 made ofan aluminum nitride (AlN) film is formed on the substrate 22 and thelower electrode 24 by sputtering an Al alloy target formed byincorporating a divalent element and a tetravalent element into Al in amixed gas atmosphere of argon and nitrogen. Instead of sputtering the Alalloy target formed by incorporating a divalent element and atetravalent element into Al, an Al target, a divalent element target,and a tetravalent element target may be simultaneously sputtered bymultiple sputtering. In this case, atomic concentrations of the divalentelement and the tetravalent element contained in the piezoelectric film26 can be controlled by changing electrical power applied to eachtarget.

As illustrated in FIG. 3C and FIG. 3G, a metal film is then formed onthe piezoelectric film 26 by sputtering in an argon gas atmosphere forexample. The metal film is selected from at least one of Al, Cu, Cr, Mo,W, Ta, Pt, Ru, Rh, and Ir as described previously. The metal film isthen shaped into a desired shape by exposure and etching to form theupper electrode 28. In addition, the piezoelectric film 26 is alsoshaped into a desired shape by exposure and etching for example.Furthermore, the hole portion 36 is formed by selectively etching thelower electrode 24 and the sacrifice layer 39.

After the above process, as illustrated in FIG. 3D and FIG. 3H, etchantis introduced from the hole portion 36 to etch the sacrifice layer 39.Here, the stress to a multilayered film formed by the lower electrode24, the piezoelectric film 26, and the upper electrode 28 is set to acompression stress by adjusting a sputtering condition. Thus, when theetching of the sacrifice layer 39 is completed, the multilayered filmbulges out, and the dome-shaped air-space 32 is formed between thesubstrate 22 and the lower electrode 24. The introduction path 34connecting the air-space 32 to the hole portion 36 is also formed. Theabove described fabrication process forms the acoustic wave deviceillustrated in FIG. 2.

A description will now be given of a simulation conducted to investigatean effective electromechanical coupling coefficient k_(eff) ²(hereinafter, referred to as k_(eff) ²) of the FBAR of the firstembodiment. The simulation uses calculated values by the first principlecalculation for the piezoelectric constant, the elastic constant, andthe permittivity of the piezoelectric film 26 made of an aluminumnitride film containing a divalent element and a tetravalent element. Adescription will now be given of a simulation performed to a first FBARand a second FBAR having the following configuration.

The first FBAR uses a multilayered metal film including Cr with a filmthickness of 100 nm and Ru with a film thickness of 225 nm stacked inthis order from the substrate 22 side for the lower electrode 24. Thepiezoelectric film 26 is an aluminum nitride film that contains Mg as adivalent element and Hf as a tetravalent element and has a filmthickness of 1000 nm. Substitutional concentrations of Mg and Hf are setto 6.25 atomic %. The upper electrode 28 is a multilayered metal filmincluding Ru with a film thickness of 225 nm and Cr with a filmthickness of 30 nm stacked in this order from the substrate 22 side. Inaddition, a silicon dioxide (SiO₂) film with a film thickness of 50 nmis located on the upper electrode 28.

The second FBAR uses an aluminum nitride film having a film thickness of1000 nm and containing Mg as a divalent element and Ti as a tetravalentelement for the piezoelectric film 26. Other configurations are the sameas those of the first FBAR. Substitutional concentrations of Mg and Tiare set to 6.25 atomic %.

For comparison, the simulation is also performed to a first comparativeexample that has the same configuration as those of the first FBAR andthe second FBAR except that a non-doped aluminum nitride film with afilm thickness of 1150 nm is used for the piezoelectric film.

FIG. 4A illustrates simulation results of a resonance characteristic ofthe first FBAR, and FIG. 4B illustrates simulation results of aresonance characteristic of the second FBAR. A solid line indicates theresonance characteristic of the first FBAR in FIG. 4A and the resonancecharacteristic of the second FBAR in FIG. 4B, and dashed lines indicatea resonance characteristic of the first comparative example. Asillustrated in FIG. 4A and FIG. 4B, an interval between a resonancefrequency and an anti-resonance frequency is wide in the first FBAR andthe second FBAR compared to that in the first comparative example.Effective electromechanical coupling coefficients k_(eff) ² of the firstFBAR, the second FBAR, and the first comparative example are 17.5%,12.9%, and 7.22%, respectively.

In addition, simulated are FBARs using various kinds of elements for thedivalent element and the tetravalent element contained in thepiezoelectric film 26 in the same manner. Table 3 presents simulationresults. Substitutional concentrations of the divalent element and thetetravalent element are set to 6.25 atomic %, and configurations otherthan the kinds of the divalent element and the tetravalent element aremade to be the same as those of the first FBAR and the second FBAR.

TABLE 3 Anti- Resonance resonance Combi- Divalent Tetravalent frequencyfrequency nation element element [MHz] [MHz] k_(eff) ² [%] Case 1 Ca Ti1928.9 2011.9 9.77 Case 2 Ca Zr 1895.8 1983.2 10.4 Case 3 Ca Hf 1875.71998.3 14.2 Case 4 Mg Ti 1930.3 2043.8 12.9 Case 5 Mg Zr 1911.9 2030.313.5 Case 6 Mg Hf 1886.9 2043.9 17.5 Case 7 Sr Hf 1901.5 1998.3 11.4Case 8 Zn Ti 1940.1 2050.4 12.6 Case 9 Zn Zr 1888.3 1995.0 12.5 Case 10Zn Hf 1887.5 2027.7 15.9 Aluminum — — 1963.0 2024.0 7.22 nitride

As presented in Table 3, the obtained results demonstrate that theacoustic wave devices using an aluminum nitride film containing adivalent element and a tetravalent element for the piezoelectric film(Case 1 through Case 10) have effective electromechanical couplingcoefficients k_(eff) ² greater than that of the acoustic wave deviceusing a non-doped aluminum nitride film for the piezoelectric film(Table 3: Aluminum nitride). A combination of the divalent element andthe tetravalent element may be Ca—Ti, Ca—Zr, Ca—Hf, Mg—Ti, Mg—Zr, Mg—Hf,Sr—Hf, Zn—Ti, Zn—Zr, or Zn—Hf as presented in Table 3, and may be othercombinations.

The first embodiment demonstrates that an acoustic wave device having ahigh electromechanical coupling coefficient can be obtained by using analuminum nitride film containing a divalent element and a tetravalentelement for the piezoelectric film 26.

The piezoelectric film 26 contains one of Ca, Mg, Sr, and Zn as thedivalent element in the simulation results presented in Table 3, but maycontain two or more of these divalent elements. Moreover, thepiezoelectric film 26 contains one of Ti, Zr, and Hf as the tetravalentelement, but may contain two or more of these tetravalent elements. Thatis to say, the piezoelectric film 26 may contain at least one of Ca, Mg,Sr, and Zn as the divalent element and at least one of Ti, Zr, and Hf asthe tetravalent element. In addition, the piezoelectric film 26 maycontain a divalent element and a tetravalent element other than thosecited in Table 3.

A description will now be given of an insulation property of doped AlNdoped with a divalent element and a tetravalent element (hereinafter,referred to as first doped AlN). The insulation property was evaluatedby calculating an electronic state of the first doped AlN by the firstprinciple calculation, and drawing a band diagram. For comparison,evaluated were an insulation property of doped AlN doped with only adivalent element (hereinafter, referred to as second doped AlN) and aninsulation property of doped AlN doped with only a tetravalent element(hereinafter, referred to as third doped AlN) in the same manner. Thefirst doped AlN, the second doped AlN, and the third doped AlN have thefollowing crystal structures.

The first doped AlN is doped AlN formed by substituting a divalentelement in one of the aluminum atoms 10 and substituting a tetravalentelement in another one of the aluminum atoms 10 in the non-doped AlNwith the wurtzite-type crystal structure described in FIG. 1. Thus, aratio of substitutional concentrations of the divalent element to thetetravalent element is 1:1. Mg is used as the divalent element, and Hfis used as the tetravalent element.

The second doped AlN is doped AlN formed by substituting a divalentelement in one of the aluminum atoms 10 in the non-doped AlN with thewurtzite-type crystal structure described in FIG. 1. Mg is used as thedivalent element.

The third doped AlN is doped AlN formed by substituting a tetravalentelement in one of the aluminum atoms 10 in the non-doped AlN with thewurtzite-type crystal structure described in FIG. 1. Hf is used as thetetravalent element.

FIG. 5 illustrates simulation results of a band structure of the firstdoped AlN. FIG. 6 illustrates simulation results of a band structure ofthe second doped AlN. FIG. 7 illustrates simulation results of a bandstructure of the third doped AlN. In FIG. 5 through FIG. 7, solid linesindicate energy levels, a band of energy levels at a lower siderepresents a valence band, and a band of energy levels at an upper siderepresents a conduction band. A forbidden band is between the valenceband and the conduction band. A dashed line indicates Fermi energy(hereinafter, abbreviated as Ef).

When AlN is doped with only Mg as a divalent element, the Fermi energyEf is located below a top of the valence band, and thus lies in thevalence band as illustrated in FIG. 6. This reveals that the insulationproperty degrades when AlN is doped with only a divalent element. WhenAlN is doped with only Hf as a tetravalent element, the Fermi energy Efis located above a bottom of the conduction band, and thus lies in theconduction band as illustrated in FIG. 7. This reveals that theinsulation property also degrades when AlN is doped with only atetravalent element.

On the other hand, when AlN is doped with Mg as a divalent element andHf as a tetravalent element at a ratio of 1:1, the Fermi energy Ef liesin the forbidden band between the top of the valence band and the bottomof the conduction band as illustrated in FIG. 5. This reveals that theinsulation property can be maintained by doping AlN with a divalentelement and a tetravalent element, and making a ratio of substitutionalconcentrations of the divalent element to the tetravalent element 1:1.This is because an electric property of the doped AlN can remain neutralby making a ratio of substitutional concentrations of the divalentelement to the tetravalent element 1:1 because both the divalent elementand the tetravalent element are substituted in trivalent aluminum sites.FIG. 5 illustrates a case where Mg is used as the divalent element andHf is used as the tetravalent element, but the insulation property canbe also maintained when other divalent elements and tetravalent elementsare used.

Therefore, an acoustic wave device that can maintain the insulationproperty of the piezoelectric film 26 and have a high electromechanicalcoupling coefficient can be obtained by using an aluminum nitride filmcontaining a divalent element and a tetravalent element at a ratio of1:1 for the piezoelectric film 26 in the FBAR of the first embodiment.The ratio of substitutional concentrations of the divalent element andthe tetravalent element is preferably 1:1 to the extent that theelectric property of the piezoelectric film can remain neutral.

Next, a description will be given of a relationship between apiezoelectric constant e₃₃ and an electromechanical coupling coefficientk² of doped AlN doped with a divalent element and a tetravalent element.The piezoelectric constant e₃₃ of the doped AlN is calculated by thefirst principle calculation, and the electromechanical couplingcoefficient k² is calculated from Expression 1. FIG. 8 illustrates arelationship between a piezoelectric constant e₃₃ and anelectromechanical coupling coefficient k² with respect to the doped AlNof Case 1 through Case 10 presented in Table 2 and the non-doped AlN. InFIG. 8, the open circle indicates a result of the non-doped AlN, andblack circles indicate results of the doped AlN. As illustrated in FIG.8, all doped AlN doped with a divalent element and a tetravalent elementhave piezoelectric constants e₃₃ greater than that of the non-doped AlN,and the electromechanical coupling coefficient k² increases as thepiezoelectric constant e₃₃ increases. This reveals that the FBAR of thefirst embodiment preferably uses an aluminum nitride film containing adivalent element and a tetravalent element and having a piezoelectricconstant e₃₃ greater than 1.55, which is the piezoelectric constant e₃₃of aluminum nitride, for the piezoelectric film 26. The aboveconfiguration allows the piezoelectric film 26 to have a highelectromechanical coupling coefficient, and accordingly allows anacoustic wave device having a high electromechanical couplingcoefficient to be obtained.

As illustrated in FIG. 8, the piezoelectric film 26 preferably has apiezoelectric constant e₃₃ greater than 1.6, more preferably 1.8 becausethe electromechanical coupling coefficient k² increases as thepiezoelectric constant e₃₃ increases.

A description will now be given of a relationship between a crystalstructure and an electromechanical coupling coefficient k² of doped AlNdoped with a divalent element and a tetravalent element. The crystalstructure of the doped AlN is evaluated with a ratio (c/a) of a latticeconstant in the c-axis direction to a lattice constant in the a-axisdirection calculated by the first principle calculation. Theelectromechanical coupling coefficient k² is calculated by assigningcalculated values of the piezoelectric constant and the like of thedoped AlN by the first principle calculation to Expression 1. FIG. 9illustrates a relationship between a c/a ratio and an electromechanicalcoupling coefficient k² with respect to the doped AlN of Case 1 throughCase 10 presented in Table 2 and the non-doped AlN. In FIG. 9, the opencircle indicates a result of the non-doped AlN, and black circlesindicate results of the doped AlN. As illustrated in FIG. 9, all dopedAlN doped with a divalent element and a tetravalent element have c/aratios less than that of the non-doped AlN, and the electromechanicalcoupling coefficient k² increases as the c/a ratio decreases. Thisreveals that the FBAR of the first embodiment preferably uses analuminum nitride film containing a divalent element and a tetravalentelement and having a c/a ratio less than 1.6, which is the c/a ratio ofaluminum nitride, for the piezoelectric film 26. The above configurationallows the piezoelectric film 26 to have a high electromechanicalcoupling coefficient, and accordingly allows an acoustic wave devicehaving a high electromechanical coupling coefficient to be obtained.

As illustrated in FIG. 9, the piezoelectric film 26 preferably has a c/aratio less than 1.595, more preferably 1.59 because theelectromechanical coupling coefficient k² increases as the c/a ratiodecreases.

A description will now be given of a dependence of an electromechanicalcoupling coefficient k² on substitutional concentrations of doped AlNdoped with a divalent element and a tetravalent element. The dependenceof the electromechanical coupling coefficient k² on substitutionalconcentrations is evaluated by calculating a size of the supercell ofthe wurtzite-type crystal structure described in FIG. 1 and electronicstates of doped AlN with different numbers of aluminum atoms substitutedby a divalent element and a tetravalent element by the first principlecalculation. Substitutional concentrations of the divalent element andthe tetravalent element are made to be equal to each other to make theelectric properties of the doped AlN neutral.

FIG. 10A illustrates a dependence of an electromechanical couplingcoefficient k² on substitutional concentrations when Mg is used as thedivalent element and Hf is used as the tetravalent element, FIG. 10Billustrates a dependence of an electromechanical coupling coefficient k²on substitutional concentrations when Mg is used as the divalent elementand Ti is used as the tetravalent element. FIG. 10A and FIG. 10B revealthat the electromechanical coupling coefficient k² of the doped AlNincreases as the substitutional concentrations increase not only when Mgand Hf are used but also when Mg and Ti are used. This result revealsthat the electromechanical coupling coefficient k² can be controlled tobe a desired value by controlling the substitutional concentrations. Forexample, doped AlN with an electromechanical coupling coefficient k² of10% can be obtained by controlling a total of substitutionalconcentrations of Mg and Hf to be approximately 4 atomic %, or bycontrolling a total of substitutional concentrations of Mg and Ti to beapproximately 7 atomic %. The simulation uses Mg as the divalent elementand Ti or Hf as the tetravalent element, but other divalent elements andtetravalent elements may be used.

Thus, an acoustic wave device having a desired electromechanicalcoupling coefficient can be obtained by controlling substitutionalconcentrations of the divalent element and the tetravalent elementcontained in the piezoelectric film 26 in the FBAR of the firstembodiment.

A description will now be given of an acoustic wave device in accordancewith a first variation of the first embodiment. FIG. 11 illustrates across-sectional view of the acoustic wave device of the first variationof the first embodiment. As illustrated in FIG. 11, an FBAR 40 of thefirst variation of the first embodiment includes a temperaturecompensation film 42 inserted so as to be sandwiched by piezoelectricfilms 26a and 26b. The temperature compensation film 42 is locatedbetween the piezoelectric films 26a and 26b, and contacts thepiezoelectric films 26a and 26b. The temperature compensation film 42 isformed of a material having a temperature coefficient of an elasticconstant opposite in sign to those of the piezoelectric films 26a and26b. For example, when the temperature coefficients of the piezoelectricfilms 26a and 26b are negative, the temperature compensation film 42with a positive temperature coefficient is used. Other configurationsare the same as those of the first embodiment, and thus a descriptionthereof is omitted.

Provision of the above described temperature compensation film 42 allowsa temperature characteristic of the FBAR 40 to be improved. A siliconoxide (SiO₂) film is an example of the temperature compensation film 42.Instead of the SiO₂ film, a film mainly containing silicon oxide, e.g. asilicon oxide film doped with an element such as fluorine (F), may beused. Here, “a film mainly containing . . . ” means a film that containsan element to the extent that the temperature coefficient of the elasticconstant of the temperature compensation film 42 becomes opposite insign to those of the piezoelectric films 26a and 26b.

A description will be given of a simulation conducted to investigate aneffective electromechanical coupling coefficient k_(eff) ² of the FBAR40 of the first variation of the first embodiment. As with the firstembodiment, the calculated values by the first principle calculation areused for the piezoelectric constants, the elastic constants, and thepermittivities of the piezoelectric films 26a and 26b that are aluminumnitride films containing a divalent element and a tetravalent element. Adescription will be given of a simulation performed to a third FBAR anda fourth FBAR having the following configurations.

The third FBAR uses a multilayered metal film including Cr with a filmthickness of 100 nm and Ru with a film thickness of 225 nm stacked inthis order from the substrate 22 side for the lower electrode 24. Thepiezoelectric films 26a and 26b are aluminum nitride films having a filmthickness of 400 nm and containing Mg as a divalent element and Hf as atetravalent element. Substitutional concentrations of Mg and Hf are setto 6.25 atomic %. A SiO₂ film with a film thickness of 50 nm is used forthe temperature compensation film 42. The upper electrode 28 is amultilayered metal film including Ru with a film thickness of 225 nm andCr with a film thickness of 30 nm stacked in this order from thesubstrate 22 side. A SiO₂ film with a film thickness of 50 nm is locatedon the upper electrode 28.

The fourth FBAR uses an aluminum nitride film having a film thickness of400 nm and containing Mg as a divalent element and Ti as a tetravalentelement for the piezoelectric films 26a and 26b. Other configurationsare the same as those of the third FBAR. Substitutional concentrationsof Mg and Ti are set to 6.25 atomic %.

For comparison, the simulation is also performed to a second comparativeexample that has the same configuration as those of the third FBAR andthe fourth FBAR except that a non-doped aluminum nitride film with afilm thickness of 475 nm is used for the piezoelectric film.

FIG. 12A illustrates simulation results of a resonance characteristic ofthe third FBAR, and FIG. 12B illustrates simulation results of aresonance characteristic of the fourth FBAR. A solid line indicates theresonance characteristic of the third FBAR in FIG. 12A and the resonancecharacteristic of the fourth FBAR in FIG. 12B, and a dashed lineindicates a resonance characteristic of the second comparative example.As illustrated in FIG. 12A and FIG. 12B, an interval between a resonancefrequency and an anti-resonance frequency is wide in the third FBAR andthe fourth FBAR compared to that in the second comparative example. Theeffective electromechanical coupling coefficients k_(eff) ² of the thirdFBAR, the fourth FBAR, and the second comparative example are 12.0%,8.78%, and 5.01%, respectively.

Also simulated are FBARs using various kinds of elements for thedivalent element and the tetravalent element contained in thepiezoelectric films 26a and 26b in the same manner. Table 4 presentssimulation results. Substitutional concentrations of the divalentelement and the tetravalent element are set to 6.25 atomic %, and theconfigurations other than the kinds of the divalent element and thetetravalent element are the same as those of the third FBAR and thefourth FBAR.

TABLE 4 Anti- Resonance resonance Combi- Divalent Tetravalent frequencyfrequency nation element element [MHz] [MHz] k_(eff) ² [%] Case 1 Ca Ti1973.7 2029.8 6.63 Case 2 Ca Zr 1948.0 2007.2 7.05 Case 3 Ca Hf 1938.82018.9 9.40 Case 4 Mg Ti 1978.5 2054.4 8.78 Case 5 Mg Zr 1964.4 2043.99.22 Case 6 Mg Hf 1949.0 2054.3 12.0 Case 7 Sr Hf 1953.8 2019.1 7.71Case 8 Zn Ti 1986.1 2059.6 8.49 Case 9 Zn Zr 1944.0 2016.4 8.53 Case 10Zn Hf 1947.5 2041.8 10.9 Aluminum — — 1965.3 2007.0 5.01 nitride

As presented in Table 4, even when the temperature compensation film 42is provided, the acoustic wave devices using an aluminum nitride filmcontaining a divalent element and a tetravalent element for thepiezoelectric film (Case 1 through Case 10) have effectiveelectromechanical coupling coefficients k_(eff) ² greater than that ofthe acoustic wave device using a non-doped aluminum nitride film for thepiezoelectric film (Table 4: Aluminum nitride). A combination of thedivalent element and the tetravalent element may be Ca—Ti, Ca—Zr, Ca—Hf,Mg—Ti, Mg—Zr, Mg—Hf, Sr—Hf, Zn—Ti, Zn—Zr, or Zn—Hf as presented in Table4, but may be other combinations.

The first variation of the first embodiment demonstrates that anacoustic wave device having a high electromechanical couplingcoefficient can be obtained by using an aluminum nitride film containinga divalent element and a tetravalent element for the piezoelectric films26a and 26b even when the temperature compensation film 42 is included.

Second Embodiment

A second embodiment is an exemplary acoustic wave device that uses analuminum nitride film containing a divalent element and a pentavalentelement for the piezoelectric film. A description will first be given ofa simulation performed to doped AlN doped with a divalent element and apentavalent element with the first principle calculation. The simulationis performed to doped AlN with a crystal structure formed bysubstituting a divalent element in two of the aluminum atoms 10 andsubstituting a pentavalent element in another one of the aluminum atoms10 in the non-doped AlN with the wurtzite-type crystal structuredescribed in FIG. 1. That is to say, a part of the aluminum sites issubstituted by a divalent element and a pentavalent element, andsimulated is the doped AlN with the wurtzite-type crystal structurecontaining thirteen aluminum atoms, two divalent elements, onepentavalent element, and sixteen nitrogen atoms. Therefore, thesubstitutional concentration of the divalent element is 12.5 atomic %,and the substitutional concentration of the pentavalent element is 6.25atomic %. Mg or Zn is used as the divalent element, and tantalum (Ta),niobium (Nb), or vanadium (V) is used as the pentavalent element.

Table 5 presents calculated values of piezoelectric constants e₃₃ andelectromechanical coupling coefficients k² calculated from Expression 1of the non-doped AlN and the doped AlN. As presented in Table 5, theobtained results demonstrate that the doped AlN doped with a divalentelement and a pentavalent element (Case 1 through Case 6) havepiezoelectric constants e₃₃ and electromechanical coupling coefficientsk² greater than those of the non-doped AlN (Table 5: Non-doped AlN). Acombination of the divalent element and the pentavalent element may beMg—Ta, Mg—Nb, Mg—V, Zn—Ta, Zn—Nb, or Zn—V as presented in Table 5, butmay be other combinations.

TABLE 5 Electro- mechanical Piezoelectric coupling Combi- DivalentPentavalent constant e₃₃ coefficient nation element element [C/m²] k²[%] Case 1 Mg Ta 2.52 19.3 Case 2 Mg Nb 2.22 14.4 Case 3 Mg V 2.33 18.1Case 4 Zn Ta 2.22 14.3 Case 5 Zn Nb 2.12 13.6 Case 6 Zn V 2.12 10.8Non-doped — — 1.55 7.12 AlN

As described above, the inventors have newly found that doped AlNcontaining a divalent element and a pentavalent element also has anelectromechanical coupling coefficient k² greater than that of non-dopedAlN. A description will now be given of the second embodiment capable ofobtaining an acoustic wave device having a high electromechanicalcoupling coefficient k² based on the above knowledge.

The acoustic wave device of the second embodiment has the sameconfiguration as that of the first embodiment except that thepiezoelectric film 26 is an aluminum nitride film containing a divalentelement and a pentavalent element, and thus a description thereof isomitted. The divalent element and the pentavalent element aresubstituted in aluminum sites of the aluminum nitride film. Thepiezoelectric film 26 has a crystal structure having a c-axisorientation as with that of the first embodiment.

A fabrication method of the acoustic wave device of the secondembodiment is the same as that of the first embodiment except that thepiezoelectric film 26 is formed with an Al alloy target formed byincorporating a divalent element and a pentavalent element into Al, andthus a description thereof is omitted. As described in the firstembodiment, the multiple sputtering technique that sputters an Altarget, a divalent element target, and a pentavalent element targetsimultaneously may be used.

A description will now be given of a simulation conducted to investigatean effective electromechanical coupling coefficient k_(eff) ² of an FBARof the second embodiment. The simulation uses calculated values by thefirst principle calculation for the piezoelectric constant, the elasticconstant, and the permittivity of the piezoelectric film 26 that is analuminum nitride film containing a divalent element and a pentavalentelement. A description will be given of a simulation performed to afifth FBAR having the following configuration.

The fifth FBAR uses a multilayered metal film including Cr with a filmthickness of 100 nm and Ru with a film thickness of 225 nm stacked inthis order from the substrate 22 side for the lower electrode 24. Thepiezoelectric film 26 is an aluminum nitride film having a filmthickness of 850 nm and containing Mg as a divalent element and Ta as apentavalent element. The substitutional concentration of Mg is set to12.5 atomic %, and the substitutional concentration of Ta is set to 6.25atomic %. The upper electrode 28 is a multilayered metal film includingRu with a film thickness of 225 nm and Cr with a film thickness of 30 nmstacked in this order from the substrate 22 side. A SiO₂ film with afilm thickness of 50 nm is located on the upper electrode 28.

FIG. 13 illustrates simulation results of a resonance characteristic ofthe fifth FBAR. A solid line indicates the resonance characteristic ofthe fifth FBAR. For comparison, a dashed line indicates the resonancecharacteristic of the first comparative example described in FIG. 4A andFIG. 4B. As illustrated in FIG. 13, an interval between a resonancefrequency and an anti-resonance frequency is wide in the fifth FBARcompared to that in the first comparative example. The effectiveelectromechanical coupling coefficient k_(eff) ² of the firstcomparative example is 7.22%, whereas the effective electromechanicalcoupling coefficient k_(eff) ² of the fifth FBAR is 17.6%.

Also simulated are FBARs using various kinds of elements for thedivalent element and the pentavalent element contained in thepiezoelectric film 26 in the same manner. Table 6 presents simulationresults. The substitutional concentration of the divalent element is setto 12.5 atomic %, the substitutional concentration of the pentavalentelement is set to 6.25 atomic %, and the configuration other than thekinds of the divalent element and the pentavalent element is the same asthat of the fifth FBAR.

TABLE 6 Anti- Resonance resonance Combi- Divalent Pentavalent frequencyfrequency nation element element [MHz] [MHz] k_(eff) ² [%] Case 1 Mg Ta1910.3 2086.1 17.6 Case 2 Mg Nb 1977.0 2107.6 14.3 Case 3 Mg V 1835.51993.4 16.8 Case 4 Zn Ta 1968.3 2096.9 14.2 Case 5 Zn Nb 1926.9 2047.113.6 Case 6 Zn V 2080.1 2179.4 10.7 Aluminum — — 1963.0 2024.0 7.22nitride

As presented in Table 6, the acoustic wave devices using an aluminumnitride film containing a divalent element and a pentavalent element forthe piezoelectric film (Case 1 through Case 6) have effectiveelectromechanical coupling coefficients k_(eff) ² greater than that ofthe acoustic wave device using a non-doped aluminum nitride film for thepiezoelectric film (Table 6: Aluminum nitride). A combination of thedivalent element and the pentavalent element may be Mg—Ta, Mg—Nb, Mg—V,Zn—Ta, Zn—Nb, or Zn—V as presented in Table 6, but may be othercombinations.

The second embodiment demonstrates that an acoustic wave device having ahigh electromechanical coupling coefficient can be also obtained byusing an aluminum nitride film containing a divalent element and apentavalent element for the piezoelectric film 26.

In Table 6, the piezoelectric film 26 contains Mg or Zn as the divalentelement, but may contain both of them. In addition, the piezoelectricfilm 26 contains one of Ta, Nb, and V as the pentavalent element, butmay contain two or more of them. That is to say, the piezoelectric film26 may contain at least one of Mg and Zn as the divalent element, andcontain at least one of Ta, Nb, and V as the pentavalent element.Furthermore, the piezoelectric film 26 may contain a divalent elementand a pentavalent element other than those cited in Table 6.

A description will now be given of an insulation property of doped AlNdoped with a divalent element and a pentavalent element (hereinafter,referred to as fourth doped AlN). The insulation property is evaluatedby calculating an electronic state of the fourth doped AlN by the firstprinciple calculation and drawing a band diagram. For comparison, aninsulation property of doped AlN doped with only a pentavalent element(hereinafter, referred to as fifth doped AlN) is also evaluated in thesame manner. The fourth doped AlN and the fifth doped AlN have thefollowing crystal structures.

The fourth doped AlN is doped AlN formed by substituting divalentelements in two of the aluminum atoms 10 and substituting a pentavalentelement in another one of the aluminum atoms 10 in the non-doped AlNwith the wurtzite-type crystal structure described in FIG. 1. Thus, aratio of the substitutional concentration of the divalent element tothat of the pentavalent element is 2:1. Mg is used as the divalentelement, and Ta is used as the pentavalent element.

The fifth doped AlN is doped AlN formed by substituting a pentavalentelement in one of the aluminum atoms 10 in the non-doped AlN with thewurtzite-type crystal structure described in FIG. 1. Ta is used as thepentavalent element.

FIG. 14 illustrates simulation results of a band structure of the fourthdoped AlN. FIG. 15 illustrates simulation results of a band structure ofthe fifth doped AlN. As described for FIG. 6, the Fermi energy Ef liesin the valence band and the insulation property degrades when AlN isdoped with only Mg as a divalent element. As illustrated in FIG. 15,when AlN is doped with only Ta as a pentavalent element, the Fermienergy Ef is located above the bottom of the conduction band, and thuslies in the conduction band. This reveals that the insulation propertyalso degrades when AlN is doped with only a pentavalent element.

On the other hand, when AlN is doped with Mg as a divalent element andTa as a pentavalent element at a ratio of 2:1, the Fermi energy Ef liesin the forbidden band between the top of the valence band and the bottomof the conduction band as illustrated in FIG. 14. This reveals that theinsulation property can be maintained by doping AlN with a divalentelement and a pentavalent element, and making a ratio of substitutionalconcentrations of the divalent element to the pentavalent element 2:1.This is because an electric property of the doped AlN can remain neutralby making a ratio of substitutional concentrations of the divalentelement to the pentavalent element 2:1 because both the divalent elementand the pentavalent element are substituted in trivalent aluminum sitesas described in the first embodiment. FIG. 14 illustrates a case whereMg is used as the divalent element and Ta is used as the pentavalentelement, but the insulation property can be maintained even when otherdivalent elements and pentavalent elements are used.

Therefore, an acoustic wave device that maintains the insulationproperty of the piezoelectric film 26 and has a high electromechanicalcoupling coefficient can be obtained by using an aluminum nitride filmcontaining a divalent element and a pentavalent element at a ratio of2:1 for the piezoelectric film 26 in the FBAR of the second embodiment.A ratio of substitutional concentrations of the divalent element to thepentavalent element is preferably 2:1 to the extent that the electricproperty of the piezoelectric film can remain neutral.

A description will be given of a relationship between a piezoelectricconstant e₃₃ and a k² of doped AlN doped with a divalent element and apentavalent element. The piezoelectric constant e₃₃ and the k² of thedoped AlN are calculated in the same way as that described in FIG. 8 ofthe first embodiment. FIG. 16 illustrates a relationship betweenpiezoelectric constants e₃₃ and k² with respect to the doped AlN of Case1 through Case 6 presented in Table 5 and the non-doped AlN. In FIG. 16,the open circle indicates a result of the non-doped AlN, and blackcircles indicate results of the doped AlN. As illustrated in FIG. 16,all doped AlN doped with a divalent element and a pentavalent elementhave piezoelectric constants e₃₃ greater than that of the non-doped AlN,and the electromechanical coupling coefficient k² increases as thepiezoelectric constant e₃₃ increases. This reveals that the FBAR of thesecond embodiment preferably uses an aluminum nitride film containing adivalent element and a pentavalent element and having a piezoelectricconstant e₃₃ greater than 1.55, which is the piezoelectric constant e₃₃of aluminum nitride, for the piezoelectric film 26 in. Theabove-described configuration allows the piezoelectric film 26 to have ahigh electromechanical coupling coefficient, and accordingly allows anacoustic wave device having a high electromechanical couplingcoefficient to be obtained.

As illustrated in FIG. 16, the piezoelectric constant e₃₃ of thepiezoelectric film 26 is preferably greater than 1.6, more preferably1.8 because the electromechanical coupling coefficient k² increases asthe piezoelectric constant e₃₃ increases.

A description will now be given of a relationship between a crystalstructure and an electromechanical coupling coefficient k² of doped AlNdoped with a divalent element and a pentavalent element. The crystalstructure of the doped AlN is evaluated with a c/a ratio as described inFIG. 9 of the first embodiment. The electromechanical couplingcoefficient k² is calculated in the same way as that described in FIG. 9of the first embodiment. FIG. 17 illustrates a relationship between ac/a ratio and an electromechanical coupling coefficient k² with respectto the doped AlN of Case 1 through Case 6 presented in Table 5 and thenon-doped AlN. In FIG. 17, the open circle indicate a result of thenon-doped AlN, and black circles indicate results of the doped AlN. Asillustrated in FIG. 17, all doped AlN doped with a divalent element anda pentavalent element have c/a ratios less than that of the non-dopedAlN, and the electromechanical coupling coefficient k² increases as thec/a ratio decreases. Therefore, the FBAR of the second embodimentpreferably uses an aluminum nitride film containing a divalent elementand a pentavalent element and having a c/a ratio less than 1.6, which isthe c/a ratio of aluminum nitride, for the piezoelectric film 26. Theabove-described configuration allows the piezoelectric film 26 to have ahigh electromechanical coupling coefficient, and thus allows an acousticwave device having a high electromechanical coupling coefficient to beobtained.

As illustrated in FIG. 17, the piezoelectric film 26 preferably has ac/a ratio less than 1.595, more preferably 1.59 because theelectromechanical coupling coefficient k² increases as the c/a ratiodecreases.

A description will be given of a dependence of an electromechanicalcoupling coefficient k² on substitutional concentrations of doped AlNdoped with a divalent element and a pentavalent element. The dependenceof the electromechanical coupling coefficient k² on substitutionalconcentrations is evaluated in the same way as that described in FIG.10A and FIG. 10B of the first embodiment. The ratio of substitutionalconcentrations of the divalent element to the pentavalent element is setto 2:1 so that the electric property of the doped AlN is neutral.

FIG. 18 illustrates a dependence of an electromechanical couplingcoefficient k² on substitutional concentrations when Mg is used as thedivalent element and Ta is used as the pentavalent element. FIG. 18reveals that the electromechanical coupling coefficient k² increases asthe substitutional concentrations increase. This reveals that theelectromechanical coupling coefficient k² of the doped AlN can becontrolled to be a desired value by controlling the substitutionalconcentrations as is the case with the first embodiment. For example,doped AlN with an electromechanical coupling coefficient k² of 10% canbe obtained by controlling the total of substitutional concentrations ofMg and Ta to be approximately 7 atomic %. The simulation uses Mg as thedivalent element and Ta as the pentavalent element, but other divalentelements and pentavalent elements may be used.

Therefore, an acoustic wave device with a desired electromechanicalcoupling coefficient can be obtained by controlling the substitutionalconcentrations of the divalent element and the pentavalent elementcontained in the piezoelectric film 26 in the FBAR of the secondembodiment.

A description will now be given of an acoustic wave device in accordancewith a first variation of the second embodiment. The acoustic wavedevice of the first variation of the second embodiment uses an aluminumnitride film containing a divalent element and a pentavalent element forthe piezoelectric films 26a and 26b. Other configurations are the sameas those of the first variation of the first embodiment, and thus adescription thereof is omitted.

A description will be given of a simulation conducted to investigate aneffective electromechanical coupling coefficient k_(eff) ² of an FBAR ofthe first variation of the second embodiment. As is the case with thesecond embodiment, calculated values by the first principle calculationare used for the piezoelectric constants, the elastic constants, and thepermittivities of the piezoelectric films 26a and 26b that are aluminumnitride films containing a divalent element and a pentavalent element. Adescription will now be given of a simulation performed to a sixth FBARhaving the following configuration.

The sixth FBAR uses a multilayered metal film including Cr with a filmthickness of 100 nm and Ru with a film thickness of 225 nm stacked inthis order from the substrate 22 side for the lower electrode 24. Thepiezoelectric films 26a and 26b are aluminum nitride films having a filmthickness of 375 nm and containing Mg as a divalent element and Ta as apentavalent element. The substitutional concentration of Mg is set to12.5 atomic %, and the substitutional concentration of Ta is set to 6.25atomic %. A SiO₂ film with a film thickness of 50 nm is used for thetemperature compensation film 42. The upper electrode 28 is amultilayered metal film including Ru with a film thickness of 225 nm andCr with a film thickness of 30 nm stacked in this order from thesubstrate 22 side. A SiO₂ film with a film thickness of 50 nm is locatedon the upper electrode 28.

FIG. 19 illustrates simulation results of a resonance characteristic ofthe sixth FBAR. A solid line indicates the resonance characteristic ofthe sixth FBAR. A dashed line indicates the resonance characteristic ofthe second comparative example described in FIG. 12 for comparison. Asillustrated in FIG. 19, an interval between a resonance frequency and ananti-resonance frequency is wide in the sixth FBAR compared to thesecond comparative example. The effective electromechanical couplingcoefficient k_(eff) ² of the second comparative example is 5.01%,whereas the effective electromechanical coupling coefficient k_(eff) ²of the sixth FBAR is 13.1%.

Also simulated are FBARs using various kinds of elements for thedivalent element and the pentavalent element contained in thepiezoelectric films 26a and 26b in the same manner. Table 7 presentssimulation results. The substitutional concentration of the divalentelement is set to 12.5 atomic %, the substitutional concentration of thepentavalent element is set to 6.25 atomic %, and the configuration otherthan the divalent element and the pentavalent element is the same asthat of the sixth FBAR.

TABLE 7 Anti- Resonance resonance Combi- Divalent Pentavalent frequencyfrequency nation element element [MHz] [MHz] k_(eff) ² [%] Case 1 Mg Ta1895.3 2008.2 13.1 Case 2 Mg Nb 1941.8 2024.8 9.69 Case 3 Mg V 1832.81936.6 12.5 Case 4 Zn Ta 1935.0 2016.7 9.59 Case 5 Zn Nb 1901.3 1978.69.26 Case 6 Zn V 2018.4 2078.8 6.96 Aluminum — — 1965.3 2007.0 5.01nitride

As presented in Table 7, the acoustic wave devices using an aluminumnitride film containing a divalent element and a pentavalent element forthe piezoelectric film (Case 1 through Case 6) have effectiveelectromechanical coupling coefficients k_(eff) ² greater than that ofthe acoustic wave device using a non-doped aluminum nitride film for thepiezoelectric film (Table 7: Aluminum nitride) even when the temperaturecompensation film 42 is included. A combination of the divalent elementand the pentavalent element may be Mg—Ta, Mg—Nb, Mg—V, Zn—Ta, Zn—Nb, orZn—V as presented in Table 7, but may be other combinations.

The first variation of the second embodiment demonstrates that anacoustic wave device having a high electromechanical couplingcoefficient can be obtained by using an aluminum nitride film containinga divalent element and a pentavalent element for the piezoelectric films26a and 26b even when the temperature compensation film 42 is included.

The first variation of the first embodiment and the first variation ofthe second embodiment insert the temperature compensation film 42between the piezoelectric films 26a and 26b, but the temperaturecompensation film 42 may be located in other locations as long as itcontacts the piezoelectric film. For example, the temperaturecompensation film 42 may be located between the upper electrode 28 andthe piezoelectric film 26b, or between the lower electrode 24 and thepiezoelectric film 26a.

Third Embodiment

A third embodiment describes an experiment performed to an aluminumnitride film formed so as to contain a divalent element and atetravalent element. The aluminum nitride film containing a divalentelement and a tetravalent element is formed as follows. Doped AlN filmswith different concentrations of Mg and Zr are formed by sputtering anAl target, a Mg target, and a Zr target simultaneously in a mixed gasatmosphere of Ar and N₂ with varying electrical power applied to eachtarget.

A description will be given of measurement results of piezoelectricconstants of the fabricated doped AlN films. A piezoelectric constant ismeasured with a piezometer under a condition that a load is 0.25N and afrequency is 110 Hz. FIG. 20A illustrates a relationship between a totalof substitutional concentrations of Mg and Zr and a normalizedpiezoelectric constant, and FIG. 20B is a diagram that extracts data ofwhich a ratio of substitutional concentrations of Mg to Zr is around 1:1from FIG. 20A. In FIG. 20A and FIG. 20B, the normalized piezoelectricconstant (vertical axis) is a piezoelectric constant normalized by thepiezoelectric constant of the non-doped AlN. Circles indicatemeasurement results of the fabricated doped AlN films. Rectanglesindicate calculation results of the first principle calculation as areference.

As illustrated in FIG. 20A and FIG. 20B, the doped AlN films containingMg and Zr have piezoelectric constants greater than that of thenon-doped AlN when they have a total of substitutional concentrations ofMg and Zr greater than or equal to 3 atomic % and less than or equal to35 atomic %. In addition, not only when the ratio of substitutionalconcentrations of Mg to Zr is around 1:1, but also when it is shiftedfrom 1:1, the piezoelectric constant is high as long as the total ofsubstitutional concentrations of Mg and Zr is greater than or equal to 3atomic % and less than or equal to 35 atomic %.

FIG. 21A illustrates a relationship between a ratio of a substitutionalconcentration of Zr to a total of substitutional concentrations of Mgand Zr and a normalized piezoelectric constant, and FIG. 21B is adiagram that extracts data of which a total of substitutionalconcentrations of Mg and Zr is greater than or equal to 3 atomic % andless than or equal to 10 atomic % from FIG. 21A. In FIG. 21A and FIG.21B, the normalized piezoelectric constant (vertical axis) is apiezoelectric constant normalized by the piezoelectric constant of thenon-doped AlN. A horizontal axis represents a ratio of a substitutionalconcentration of Zr to a total of substitutional concentrations of Mgand Zr (substitutional concentration of Zr/(total of substitutionalconcentrations of Mg and Zr)).

As illustrated in FIG. 21A and FIG. 21B, the doped AlN films containingMg and Zr have piezoelectric constants greater than that of thenon-doped AlN when they have a ratio of a substitutional concentrationof Zr to a total of substitutional concentrations of Mg and Zr greaterthan or equal to 0.35 and less than or equal to 0.75. In addition, whenthe total of substitutional concentrations of Mg and Zr is greater thanor equal to 3 atomic % and less than or equal to 10 atomic %, thepiezoelectric constant is almost constant as long as the ratio of asubstitutional concentration of Zr to a total of substitutionalconcentrations of Mg and Zr is greater than or equal to 0.35 and lessthan or equal to 0.75.

Here, a description will be given of a dependence of a piezoelectricconstant on substitutional concentrations of doped AlN doped with Mg orZn as a divalent element and Hf, Ti, or Zr as a tetravalent element. Thedependence of the piezoelectric constant on substitutionalconcentrations is evaluated by calculation by the first principlecalculation. FIG. 22A and FIG. 22B illustrate relationships between atotal of substitutional concentrations of a divalent element and atetravalent element and a normalized piezoelectric constant. In FIG. 22Aand FIG. 22B, the normalized piezoelectric constant (vertical axis) is apiezoelectric constant normalized by the piezoelectric constant of thenon-doped AlN. FIG. 22A illustrates cases where AlN is doped with Mg asa divalent element and Hf, Ti, or Zr as a tetravalent element, and FIG.22B illustrates cases where AlN is doped with Zn as a divalent elementand Hf, Ti, or Zr as a tetravalent element.

As illustrated in FIG. 22A and FIG. 22B, the piezoelectric constantmonotonically increases with increase in substitutional concentrationswhether Mg or Zn is used as the divalent element and Hf, Ti, or Zr isused as the tetravalent element to dope AlN. This result dictates thatthe same tendency will be obtained when other elements are used althoughFIG. 20A through FIG. 21B illustrate measurement results when Mg is usedas the divalent element and Zr is used as the tetravalent element.

Therefore, when an aluminum nitride film containing a divalent elementand a tetravalent element is used for a piezoelectric film in anacoustic wave device, a total of substitutional concentrations of thedivalent element and the tetravalent element is preferably greater thanor equal to 3 atomic % and less than or equal to 35 atomic % asillustrated in FIG. 20A and FIG. 20B. The above-described configurationcan make the piezoelectric constant of the piezoelectric film large, andthus allows the acoustic wave device to have a high electromechanicalcoupling coefficient. To make the piezoelectric constant of thepiezoelectric film large, the total of substitutional concentrations ofthe divalent element and the tetravalent element is preferably greaterthan or equal to 5 atomic % and less than or equal to 35 atomic %, andmore preferably greater than or equal to 10 atomic % and less than orequal to 35 atomic %.

As illustrated in FIG. 21A and FIG. 21B, the ratio of the substitutionalconcentration of the tetravalent element to the total of substitutionalconcentrations of the divalent element and the tetravalent element ispreferably greater than or equal to 0.35 and less than or equal to 0.75.The above configuration can make the piezoelectric constant of thepiezoelectric film large, and allows the acoustic wave device to have ahigh electromechanical coupling coefficient. To maintain the insulationproperty of the piezoelectric film, the ratio of the substitutionalconcentration of the tetravalent element to the total of substitutionalconcentrations of the divalent element and the tetravalent element ispreferably greater than or equal to 0.4 and less than or equal to 0.6,and more preferably greater than or equal to 0.45 and less than or equalto 0.55, and further preferably equal to 0.5.

A description will now be given of measurement results of a ratio (c/a)of a lattice constant in the c-axis direction to a lattice constant inthe a-axis direction in the fabricated doped AlN films. FIG. 23illustrates a relationship between a total of substitutionalconcentrations of Mg and Zr and a c/a ratio. Circles indicatemeasurement results of the fabricated doped AlN films. For comparison, arectangle indicates a calculation result of c/a of non-doped AlN by thefirst principle calculation. FIG. 23 demonstrates that the doped AlNfilms containing Mg and Zr have c/a ratios less than that of thenon-doped AlN when they have a total of substitutional concentrations ofMg and Zr greater than or equal to 3 atomic % and less than or equal to35 atomic %.

Thus, the total of substitutional concentrations of the divalent elementand the tetravalent element is preferably greater than or equal to 3atomic % and less than or equal to 35 atomic % to make the c/a ratio ofthe piezoelectric film small and the electromechanical couplingcoefficient of the acoustic wave device high.

Fourth Embodiment

A fourth embodiment first describes a relationship between a resonancefrequency and a size of a resonance portion of an acoustic wave device.For example, an acoustic wave device with an impedance of 50Ω has arelationship between a resonance frequency fr and a capacitance Cexpressed with fr=1/(2π×C×50). As described above, the capacitanceincreases as the resonance frequency becomes lower in the acoustic wavedevice. The capacitance is proportional to an area of the resonanceportion, and accordingly the resonance portion becomes larger as theresonance frequency becomes lower. In addition, a frequency f and awavelength λ of an acoustic wave have a relationship of f=V/λ. Vrepresents the acoustic velocity of the acoustic wave. The wavelength λis equal to a period of a comb-shaped electrode when a surface acousticwave is used, and is equal to the double of total film thickness of amultilayered film of the resonance portion when a bulk acoustic wave isused. The acoustic velocity V of the acoustic wave depends on a materialto be used, and accordingly, the wavelength becomes longer and theresonance portion becomes larger as the resonance frequency becomeslower.

A description will now be given of a simulation conducted to investigatea relationship between a resonance frequency and a size of a resonanceportion in an acoustic wave device. The simulation is performed to anFBAR of a third comparative example that uses a non-doped AlN film forthe piezoelectric film 26 in the FBAR having the structure illustratedin FIG. 2A through FIG. 2C of the first embodiment. The non-doped AlN isassumed to have a permittivity ∈₃₃ of 8.42×10⁻ ₁₁ F/m, and an acousticvelocity V of 11404 m/s. These values are calculated by the firstprinciple calculation. A resonance frequency is 2 GHz when Ru with athickness of 240 nm is used for the lower electrode 24 and the upperelectrode 28 and a non-doped AlN film with a thickness of 1300 nm isused for the piezoelectric film 26 to configure the resonance portion30. An area of the resonance portion 30 is 2.455×10⁻⁸ m² when the FBARhas an impedance of 50Ω. Here, to investigate a relationship between aresonance frequency and a film thickness of the resonance portion, aresonance frequency is varied by changing a total film thickness withkeeping a ratio of film thicknesses of the lower electrode 24, thepiezoelectric film 26, and the upper electrode 28 the same. In addition,to investigate a relationship between a resonance frequency and an areaof the resonance portion, the area of the resonance portion 30 ischanged so that the FBAR has an impedance of 50Ω at each resonancefrequency.

FIG. 24A illustrates a relationship between a resonance frequency and anormalized film thickness of a resonance portion of the FBAR of thethird comparative example, and FIG. 24B illustrates a relationshipbetween a resonance frequency and a normalized area of the resonanceportion. In FIG. 24A and FIG. 24B, the normalized film thickness and thenormalized area (vertical axis) are a film thickness and an areanormalized by a film thickness and an area when a resonance frequency is2 GHz, respectively. As illustrated in FIG. 24A and FIG. 24B, a filmthickness and area of the resonance portion become larger as theresonance frequency becomes lower. As described above, the acoustic wavedevice grows in size as the resonance frequency becomes lower.Especially, when the resonance frequency is less than or equal to 1.5GHz, the acoustic wave device drastically grows in size, and when theresonance frequency is less than or equal to 1.0 GHz, the acoustic wavedevice further drastically grows in size.

As described above, a capacitance increases as a resonance frequencybecomes lower. The capacitance is proportional to an area of theresonance portion of the acoustic wave device, and is also proportionalto a permittivity of the piezoelectric film used in the acoustic wavedevice. Therefore, use of a piezoelectric film with a high permittivityin the acoustic wave device can reduce an area of the resonance portionto obtain a desired capacitance and prevent the acoustic wave devicefrom growing in size. Moreover, the above described relationalexpression of f=V/λ suggests that use of a piezoelectric film with a lowacoustic velocity can shorten the wavelength λ to obtain a desiredfrequency f, and prevent the acoustic wave device from growing in size.Accordingly, a description will now be given of a simulation conductedto obtain a piezoelectric film having a high permittivity and a lowacoustic velocity.

The simulation is performed to doped AlN with a crystal structure formedby substituting a trivalent element in one of the aluminum atoms 10 inthe non-doped AlN with the wurtzite-type crystal structure illustratedin FIG. 1 of the first embodiment. That is to say, simulated is thedoped AlN having the wurtzite-type crystal structure containing fifteenaluminum atoms, one trivalent element, and sixteen nitrogen atoms. Thetrivalent element has a substitutional concentration of 6.25 atomic %.Scandium (Sc) or yttrium (Y) is used as the trivalent element. Inaddition, as is the case with the first embodiment, doped AlN doped witha divalent element and a tetravalent element is also simulated. Thedivalent element and the tetravalent element have substitutionalconcentrations of 6.25 atomic %. Ca, Mg, Sr, or Zn is used as thedivalent element, and Ti, Zr, or Hf is used as the tetravalent element.

Table 8 presents calculated values of permittivities ∈₃₃ in the c-axisdirection and acoustic velocities V of the non-doped AlN and the dopedAlN. As presented in Table 8, the doped AlN doped with a trivalentelement (Case 1 and Case 2) and the doped AlN doped with a divalentelement and a tetravalent element (Case 3 through Case 14) have highpermittivities ∈₃₃ and low acoustic velocities V compared to thenon-doped AlN (Table 8: Non-doped AlN). The trivalent element, thedivalent element, and the tetravalent element are not limited to thosepresented in Table 8, and may be other elements.

TABLE 8 Acoustic Di- Tri- Tetra- Permittivity velocity Combi- valentvalent valent ϵ₃₃ V nation element element element [×10⁻¹¹ F/m] [m/s]Case 1 — Sc — 8.96 10815 Case 2 — Y — 8.92 10914 Case 3 Ca — Ti 9.5810182 Case 4 Ca — Zr 10.10 10026 Case 5 Ca — Hf 9.98 10108 Case 6 Mg —Ti 9.71 10357 Case 7 Mg — Zr 9.77 10283 Case 8 Mg — Hf 9.79 10358 Case 9Sr — Ti 10.10 10315 Case 10 Sr — Zr 10.70 10089 Case 11 Sr — Hf 10.2010108 Case 12 Zn — Ti 9.82 10393 Case 13 Zn — Zr 9.78 10090 Case 14 Zn —Hf 9.85 10269 Non-doped — — — 8.42 11404 AlN

As presented previously, the inventors have found that the doped AlNcontaining a trivalent element and the doped AlN containing a divalentelement and a tetravalent element have high permittivities ∈₃₃ and lowacoustic velocities V compared to the non-doped AlN. Here, a descriptionwill be given of dependences of a permittivity ∈₃₃ and an acousticvelocity V on a substitutional concentration of doped AlN doped with atrivalent element. The dependences of the permittivity ∈₃₃ and theacoustic velocity V on the substitutional concentration are evaluated bycalculation by the first principle calculation using Sc as the trivalentelement. FIG. 25A illustrates a relationship between a substitutionalconcentration of Sc and a permittivity ∈₃₃, and FIG. 25B illustrates arelationship between a substitutional concentration of Sc and anacoustic velocity V. FIG. 25A and FIG. 25B demonstrate that thepermittivity ∈₃₃ increases and the acoustic velocity V decreases withincrease in the substitutional concentration of Sc. The result havingthe same tendency is obtained not only when AlN is doped with Sc, butalso when it is doped with the trivalent element, or the divalentelement and the tetravalent element presented in Table 8. As describedabove, the concentration of the element with which AlN is doped canchange the permittivity ∈₃₃ and the acoustic velocity V of the dopedAlN. Thus, based on the above described knowledge, a description will begiven of an acoustic wave device that is prevented from growing in sizeeven when the resonance frequency is less than or equal to 1.5 GHz.

An acoustic wave device of the fourth embodiment has the sameconfiguration as that illustrated in FIG. 2A through FIG. 2C of thefirst embodiment except that the piezoelectric film 26 is an aluminumnitride film containing a trivalent element, and thus a descriptionthereof is omitted. The piezoelectric film 26 has a crystal structurewith a c-axis orientation.

A description will be given of a simulation conducted to investigate arelationship between a resonance frequency and a size of a resonanceportion of an FBAR of the fourth embodiment. Simulated is an FBAR usingRu for the lower electrode 24 and the upper electrode 28, and analuminum nitride film containing Sc with a substitutional concentrationof 30 atomic % for the piezoelectric film 26. As with the simulationdescribed in FIG. 24A and FIG. 24B, the resonance frequency is varied toinvestigate the relationship between the resonance frequency and thefilm thickness of the resonance portion 30 by changing a total filmthickness with keeping a ratio of film thicknesses of the lowerelectrode 24, the piezoelectric film 26, and the upper electrode 28 thesame. In addition, the relationship between the resonance frequency andthe area of the resonance portion 30 is investigated by varying the areaof the resonance portion 30 so that the FBAR has an impedance of 50Ω ateach resonance frequency. The doped AlN doped with Sc with asubstitutional concentration of 30 atomic % is assumed to have apermittivity ∈₃₃ of 1.18×10⁻¹⁰ F/m and an acoustic velocity V of 8646m/s. These values are calculated values by the first principlecalculation.

FIG. 26A illustrates a relationship between a resonance frequency and anormalized film thickness of a resonance portion of the FBAR of thefourth embodiment, and FIG. 26B illustrates a relationship between aresonance frequency and a normalized area of the resonance portion.Solid lines indicate simulation results of the fourth embodiment, andfor comparison, dashed lines indicate simulation results of the thirdcomparative example. In FIG. 26A and FIG. 26B, the normalized filmthickness and the normalized area (vertical axis) are a film thicknessand an area normalized by a film thickness and an area when a resonancefrequency is 2 GHz in the FBAR of the third comparative example,respectively. As illustrated in FIG. 26A and FIG. 26B, at the sameresonance frequency, the resonance portion 30 of the fourth embodimenthas a smaller film thickness and a smaller area than that of the thirdcomparative example. For example, when the resonance frequency is 700MHz, the normalized film thickness of the resonance portion is 2.84 inthe third comparative example, whereas that of the fourth embodiment is2.15 and reduced by approximately 24%. The normalized area of theresonance portion is 8.07 in the third comparative example, whereas thatof the fourth embodiment is 4.40 and reduced by about 45%.

The fourth embodiment uses an aluminum nitride film containing atrivalent element that increases a permittivity ∈₃₃ and decreases anacoustic velocity V for the piezoelectric film 26. This configurationallows the resonance portion to have a small film thickness and a smallarea as illustrated in FIG. 26A and FIG. 26B, and can prevent theacoustic wave device from growing in size even when the acoustic wavedevice has a resonance frequency less than or equal to 1.5 GHz.

As presented in Table 8, the permittivity ∈₃₃ increases and the acousticvelocity V decreases when AlN is doped with a divalent element and atetravalent element. Therefore, an aluminum nitride film containing adivalent element and a tetravalent element may be used for thepiezoelectric film 26. When an aluminum nitride film containing thetrivalent element presented in Table 8 is used for the piezoelectricfilm 26, at least one of Sc and Y may be contained. When an aluminumnitride film containing the divalent element and the tetravalent elementpresented in Table 8 is used, at least one of Ca, Mg, Sr, and Zn may becontained as the divalent element, and at least one of Ti, Zr, and Hfmay be contained as the tetravalent element.

The acoustic wave device can be prevented from growing in size byachieving at least one of an increase in the permittivity ∈₃₃ and adecrease in the acoustic velocity V in the piezoelectric film 26.Therefore, the piezoelectric film 26 is not limited to an aluminumnitride film containing a trivalent element, or a divalent element and atetravalent element, and may be an aluminum nitride film containing anelement that can achieve at least one of an increase in a permittivity∈₃₃ and a decrease in an acoustic velocity V. Moreover, when a trivalentelement, or a divalent element and a tetravalent element are contained,elements other than the elements presented in Table 8 may be contained.

When the resonance frequency is less than or equal to 1.5 GHz, theacoustic wave device drastically grows in size, and when the resonancefrequency is less than or equal to 1.0 GHz, the acoustic wave devicefurther drastically grows in size. This fact leads a conclusion that analuminum nitride film containing an element that contributes to at leastone of an increase in the permittivity ∈₃₃ and a decrease in theacoustic velocity V is preferably used for the piezoelectric film 26 ofthe acoustic wave device with a resonance frequency less than or equalto 1.0 GHz.

To prevent the acoustic wave device from growing in size, thepermittivity ∈₃₃ of the piezoelectric film 26 is preferably greater than8.42×10⁻¹¹ F/m, which is the permittivity of the non-doped AlN. Theacoustic velocity V is preferably less than 11404 m/s, which is theacoustic velocity of the non-doped AlN.

A description will now be given of acoustic wave devices in accordancewith a first variation and a second variation of the fourth embodiment.FIG. 27A is a cross-sectional view of the acoustic wave device inaccordance with the first variation of the fourth embodiment, and FIG.27B is a cross-sectional view of the acoustic wave device in accordancewith the second variation of the fourth embodiment. As illustrated inFIG. 27A, an FBAR of the first variation of the fourth embodimentincludes the temperature compensation film 42 located between thepiezoelectric film 26 and the upper electrode 28 and contacting thepiezoelectric film 26 and the upper electrode 28. An aluminum nitridefilm containing a trivalent element is used for the piezoelectric film26. Other configurations are the same as those of the first variation ofthe first embodiment, and thus a description thereof is omitted.

As illustrated in FIG. 27B, an FBAR of the second variation of thefourth embodiment includes the upper electrode 28 including a lowerlayer 28a and an upper layer 28b. The temperature compensation film 42is located between the lower layer 28a and the upper layer 28b. Asdescribed above, the upper electrodes 28 are formed on the top surfaceand the bottom surface of the temperature compensation film 42, andmutually electrically short-circuited. Accordingly, a capacitance of thetemperature compensation film 42 fails to electrically contribute, andthe effective electromechanical coupling coefficient can be made high.An aluminum nitride film containing a trivalent element is used for thepiezoelectric film 26 as is the case with the first variation of thefourth embodiment. Other configurations are the same as those of thefirst variation of the first embodiment, and thus a description thereofis omitted.

A description will be given of a simulation conducted to investigate arelationship between a resonance frequency and a size of a resonanceportion in the FBARs of the first variation and the second variation ofthe fourth embodiment. Simulated is an FBAR using Ru for the lowerelectrode 24 and the upper electrode 28, an aluminum nitride filmcontaining Sc with a substitutional concentration of 30 atomic % for thepiezoelectric film 26, and a SiO₂ film for the temperature compensationfilm 42.

In the first variation of the fourth embodiment, the resonance frequencyis 2 GHz when the lower electrode 24 has a thickness of 160 nm, thepiezoelectric film 26 has a thickness of 870 nm, the temperaturecompensation film 42 has a thickness of 100 nm, and the upper electrode28 has a thickness of 160 nm. In addition, the area of the resonanceportion 30 is 1.595×10⁻⁸ m² when the FBAR has an impedance of 50Ω.

In the second variation of the fourth embodiment, the resonancefrequency is approximately 40 MHz lower than that in the first variationof the fourth embodiment when the lower electrode 24 has a thickness of160 nm, the piezoelectric film 26 has a thickness of 870 nm, thetemperature compensation film 42 has a thickness of 100 nm, the lowerlayer 28a of the upper electrode 28 has a thickness of 20 nm, the upperlayer 28b has a thickness of 160 nm.

As with the simulation described in FIG. 24A and FIG. 24B, the resonancefrequency is varied to investigate a relationship between the resonancefrequency and the film thickness of the resonance portion 30 by changinga total film thickness with keeping a ratio of film thicknesses oflayers constituting the resonance portion 30 the same. In addition, thearea of the resonance portion 30 is varied so that the FBAR has animpedance of 50Ω at each resonance frequency in order to investigate arelationship between the resonance frequency and the area of theresonance portion 30.

FIG. 28A illustrates relationships between a resonance frequency and anormalized film thickness of a resonance portion in the first variationand the second variation of the fourth embodiment, and FIG. 28Billustrates relationships between a resonance frequency and a normalizedarea of the resonance portion. In FIG. 28A and FIG. 28B, the normalizedfilm thickness and the normalized area (vertical axis) are a filmthickness and an area normalized by a film thickness and an area when aresonance frequency is 2 GHz in the FBAR of the first variation of thefourth embodiment, respectively. Solid lines indicate simulation resultsof the first variation of the fourth embodiment, and dashed linesindicate simulation results of the second variation of the fourthembodiment. FIG. 28A and FIG. 28B demonstrate that the film thickness ofthe resonance portion 30 is almost the same in the first variation andthe second variation of the fourth embodiment at the same resonancefrequency, but the area of the resonance portion 30 is small in thesecond variation of the fourth embodiment compared to that in the firstvariation. For example, when the resonance frequency is 700 MHz, thenormalized area of the resonance portion is 8.20 in the first variationof the fourth embodiment, whereas that of the second variation of thefourth embodiment is 5.90 and reduced by approximately 28%.

The first variation of the fourth embodiment demonstrates that an effectof temperature compensation can be obtained and the acoustic wave devicecan be prevented from growing in size by using an aluminum nitride filmcontaining an element that achieves at least one of an increase in thepermittivity ∈₃₃ and a decrease in the acoustic velocity V for thepiezoelectric film 26, and including the temperature compensation film42. The second variation of the fourth embodiment demonstrates that botha temperature compensation and an increase in the effectiveelectromechanical coupling coefficient can be achieved and the acousticwave device can be prevented from growing in size by includingconductive films that are formed on the top surface and the bottomsurface of the temperature compensation film 42 and mutuallyshort-circuited.

In the first variation of the fourth embodiment, the temperaturecompensation film 42 may be inserted into the piezoelectric film 26, ormay be located between the lower electrode 24 and the piezoelectric film26. In addition, the second variation of the fourth embodiment uses theupper electrodes 28 as conductive films that are formed on the topsurface and the bottom surface of the temperature compensation film 42and mutually short-circuited, but may use the lower electrode 24. Whenthe temperature compensation film 42 is inserted into the piezoelectricfilm 26, new conductive films that are mutually electricallyshort-circuited may be formed on the top surface and the bottom surfaceof the temperature compensation film 42.

As illustrated in FIG. 2B, the first embodiment through the fourthembodiment describes the air-space 32 formed by a dome-shaped bulgebetween the substrate 22 and the lower electrode 24, but the air-space32 may have a structure illustrated in FIG. 29A through FIG. 29B. FIG.29A illustrates a cross-section of an FBAR of a first variation of theembodiments, and FIG. 29B illustrates a cross-section of an FBAR of asecond variation of the embodiments. As illustrated in FIG. 29A, anair-space 32a is formed by removing a part of the substrate 22 below thelower electrode 24 in the resonance portion 30 in the FBAR of the firstvariation of the embodiments. As illustrated in FIG. 29B, an air-space32b is formed so that it pierces through the substrate 22 below thelower electrode 24 in the resonance portion 30 in the FBAR of the secondvariation of the embodiments.

In addition, the acoustic wave device is not limited to a piezoelectricthin film resonator of FBAR type, and may be a piezoelectric thin filmresonator of SMR (Solidly Mounted Resonator) type. FIG. 29C illustratesa cross-section of an SMR. As illustrated in FIG. 29C, the SMR includesan acoustic reflection film 50 formed by alternately stacking a film 52having a high acoustic impedance and a film 54 having a low acousticimpedance with a film thickness of λ/4 (λ is the wavelength of theacoustic wave) under the lower electrode 24.

Furthermore, the acoustic wave device may be a piezoelectric thin filmresonator of CRF (Coupled Resonator Filter) type. FIG. 30 illustrates across-section of a CRF. As illustrated in FIG. 30, the CRF includes afirst piezoelectric thin film resonator 92 and a second piezoelectricthin film resonator 94 stacked on the substrate 22. The firstpiezoelectric thin film resonator 92 includes the lower electrode 24,the piezoelectric film 26, and the upper electrode 28. The secondpiezoelectric thin film resonator 94 includes the lower electrode 24,the piezoelectric film 26, and the upper electrode 28. A decoupler film90 with a single layer is located between the upper electrode 28 of thefirst piezoelectric thin film resonator 92 and the lower electrode 24 ofthe second piezoelectric thin film resonator 94. The decoupler film 90may be a film containing silicon oxide, such as a silicon oxide film orsilicon oxide film containing an additive element.

The acoustic wave device may be a surface acoustic wave device or Lambwave device. FIG. 31A is a top view of a surface acoustic wave device,and FIG. 31B is a cross-sectional view taken along line A-A in FIG. 31A.FIG. 31C is a cross-sectional view of a Love wave device, and FIG. 31Dis a cross-sectional view of a boundary acoustic wave device. Asillustrated in FIG. 31A and FIG. 31B, a piezoelectric film 62 is formedon a support substrate 60 made of an insulative substrate such as a Sisubstrate, a glass substrate, a ceramic substrate, or a sapphiresubstrate. The piezoelectric film 62 is an aluminum nitride filmcontaining a divalent element and a tetravalent element, an aluminumnitride film containing a divalent element and a pentavalent element, oran aluminum nitride film containing a trivalent element or otherelements, and is made of the same material as that of the piezoelectricfilm 26 described in the first embodiment and the second embodiment, orthe fourth embodiment. A metal film 64 such as Al or Cu is formed on thepiezoelectric film 62. The metal film 64 forms reflectors R0, an IDT(Interdigital Transducer) IDT0, an input terminal T_(in), and an outputterminal T_(out). The IDT0 includes two comb-shaped electrodes 66. Oneof the comb-shaped electrodes 66 is coupled to the input terminalT_(in), and the other one is coupled to the output terminal T_(out). Theinput terminal T_(in) and the output terminal T_(out) form externalconnection terminals. The reflectors R0 are located at both sides of theIDT0 in the propagation direction of the acoustic wave. The comb-shapedelectrodes 66 and the reflectors R0 include electrode fingers arrangedat intervals corresponding to the wavelength λ of the acoustic wave. Theacoustic wave excited by the IDT0 propagates through the surface of thepiezoelectric film 62, and is reflected by the reflectors R0. Thisallows the surface acoustic wave device to resonate at a frequencycorresponding to the wavelength λ of the acoustic wave. That is to say,the comb-shaped electrodes 66 located on the piezoelectric film 62function as electrodes that excite the acoustic wave propagating throughthe piezoelectric film 62.

Plan views of the Love wave device and the boundary acoustic wave deviceare the same as FIG. 31A, and thus a description thereof is omitted. TheLove wave device includes a dielectric film 68 formed so as to cover themetal film 64 and contact the top surface of the piezoelectric film 62as illustrated in FIG. 31C. The dielectric film 68 can function as atemperature compensation film when the dielectric film 68 is formed of amaterial having a temperature coefficient of an elastic constantopposite in sign to that of the piezoelectric film 62. The dielectricfilm 68 may be a film mainly containing silicon oxide such as SiO₂. Theboundary acoustic wave device further includes a dielectric film 70formed on the dielectric film 68 as illustrated in FIG. 31D. Thedielectric film 70 may be an aluminum oxide film for example. To confinethe acoustic wave in the dielectric film 68, the dielectric film 70 hasan acoustic velocity faster than that in the dielectric film 68.

FIG. 31A through FIG. 31D illustrate the piezoelectric film 62 locatedon the support substrate 60, but the piezoelectric film 62 may be madeto have a large thickness so that it has a supporting function as asubstrate instead of the support substrate 60.

FIG. 32 is a cross-sectional view of a Lamb wave device. The Lamb wavedevice includes a second support substrate 82 located on a first supportsubstrate 80. The second support substrate 82 is bonded to the topsurface of the first support substrate 80 by surface activated bondingor resin bonding for example. The first support substrate 80 and thesecond support substrate 82 may be an insulative substrate such as a Sisubstrate, a glass substrate, a ceramic substrate, or a sapphiresubstrate. A piezoelectric film 84 is located on the second supportsubstrate 82. The piezoelectric film 84 is an aluminum nitride filmcontaining a divalent element and a tetravalent element, an aluminumnitride film containing a divalent element and a pentavalent element, oran aluminum nitride film containing a trivalent element or otherelements, and is made of the same material as that of the piezoelectricfilm 26 described in the first embodiment and the second embodiment, orthe fourth embodiment. Hole portions piercing through the second supportsubstrate 82 in a thickness direction are formed, and the hole portionsfunction as air-spaces 86 between the first support substrate 80 and thepiezoelectric film 84. An electrode 88 is located on the piezoelectricfilm 84 and in a region located above the air-spaces 86. The electrode88 is an IDT, and reflectors (not illustrated) are located at both sidesof the IDT. The acoustic wave excited by the electrode 88 is repeatedlyreflected between the top and bottom surface of the piezoelectric film84, and propagated through the piezoelectric film 84 in a lateraldirection.

The Lamb wave device may also include a dielectric film formed so as tocover the electrode 88 and contact the top surface of the piezoelectricfilm 84 as illustrated in FIG. 31C. The dielectric film can function asa temperature compensation film when it is formed of a material having atemperature coefficient of an elastic constant opposite in sign to thatof the piezoelectric film 84.

Although the embodiments of the present invention have been described indetail, it is to be understood that the various change, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

What is claimed is:
 1. An acoustic wave device comprising: apiezoelectric film made of an aluminum nitride film containing adivalent element and one of a tetravalent or pentavalent element; andelectrodes connected to the piezoelectric film to excite an acousticwave propagating through the piezoelectric film, wherein the divalentelement and one of the tetravalent or pentavalent element aresubstituted for aluminum atoms of the aluminum nitride film.
 2. Theacoustic wave device according to claim 1, wherein the piezoelectricfilm is the aluminum nitride film containing the divalent element andthe tetravalent element, and contains at least one of titanium,zirconium, and hafnium as the tetravalent element.
 3. The acoustic wavedevice according to claim 2, wherein the piezoelectric film contains atleast one of calcium, magnesium, strontium, and zinc as the divalentelement.
 4. The acoustic wave device according to claim 1, wherein thepiezoelectric film is the aluminum nitride film containing the divalentelement and the pentavalent element, and contains at least one oftantalum, niobium, and vanadium as the pentavalent element.
 5. Theacoustic wave device according to claim 4, wherein the piezoelectricfilm contains at least one of magnesium and zinc as the divalentelement.
 6. The acoustic wave device according to claim 1, wherein thepiezoelectric film has a piezoelectric constant e₃₃ greater than 1.55C/m².
 7. The acoustic wave device according to claim 1, wherein thepiezoelectric film has a ratio of a lattice constant in a c-axisdirection to a lattice constant in an a-axis direction less than 1.6. 8.The acoustic wave device according to claim 1, wherein the piezoelectricfilm is the aluminum nitride film containing the divalent element andthe tetravalent element, and a total of concentrations of the divalentelement and the tetravalent element is greater than or equal to 3 atomic% and less than or equal to 35 atomic % when a total number of atoms ofthe divalent element, the tetravalent element, and aluminum in thealuminum nitride film defines 100 atomic %.
 9. The acoustic wave deviceaccording to claim 1, wherein the piezoelectric film is the aluminumnitride film containing the divalent element and the tetravalentelement, and a ratio of a concentration of the tetravalent element to atotal of concentrations of the divalent element and the tetravalentelement is greater than or equal to 0.35 and less than or equal to 0.75when a total number of atoms of the divalent element, the tetravalentelement, and aluminum in the aluminum nitride film defines 100 atomic %.10. The acoustic wave device according to claim 1, wherein thepiezoelectric film has a crystal structure with a c-axis orientation.11. The acoustic wave device according to claim 1, further comprising atemperature compensation film having a temperature coefficient of anelastic constant opposite in sign to a temperature coefficient of anelastic constant of the piezoelectric film.
 12. The acoustic wave deviceaccording to claim 11, wherein the temperature compensation filmcontacts the piezoelectric film.
 13. The acoustic wave device accordingto claim 11, wherein the temperature compensation film contains mainlysilicon oxide.
 14. The acoustic wave device according to claim 1,wherein the electrode includes an upper electrode and a lower electrodefacing each other across the piezoelectric film.
 15. The acoustic wavedevice according to claim 14, further comprising: a temperaturecompensation film having a temperature coefficient of an elasticconstant opposite in sign to a temperature coefficient of an elasticconstant of the piezoelectric film; and conductive films that arelocated on a top surface and a bottom surface of the temperaturecompensation film and electrically connected to each other.
 16. Theacoustic wave device according to claim 1, further comprising: a firstpiezoelectric thin film resonator and a second piezoelectric thin filmresonator, each including the piezoelectric film and the electrode thatincludes an upper electrode and a lower electrode facing each otheracross the piezoelectric film, wherein the first piezoelectric thin filmresonator and the second piezoelectric thin film resonator are stacked,and a decoupler film is located between the upper electrode included inthe first piezoelectric thin film resonator and the lower electrodeincluded in the second piezoelectric thin film resonator.
 17. Theacoustic wave device according to claim 1, wherein the electrode is acomb-shaped electrode located on the piezoelectric film.
 18. Theacoustic wave device according to claim 17, wherein the acoustic waveexcited by the electrode when the element is added to the aluminumnitride constituting the film which is then charged with electricity isa surface acoustic wave or a Lamb wave.
 19. An acoustic wave devicecomprising: a piezoelectric film made of an aluminum nitride filmcontaining an element capable of at least increasing a permittivity ordecreasing an acoustic velocity when the element is added to thealuminum nitride constituting the film which is then charged withelectricity; and electrodes connected to the piezoelectric film toexcite an acoustic wave propagating through the piezoelectric film,wherein a resonance frequency of the acoustic wave is less than or equalto 1.5 GHz, and the element is substituted for aluminum atoms of thealuminum nitride film.
 20. The acoustic wave device according to claim19, wherein the piezoelectric film has a permittivity ∈₃₃ greater than8.42×10⁻¹¹ F/m.
 21. The acoustic wave device according to claim 19,wherein the piezoelectric film has an acoustic velocity less than 11404m/s.
 22. The acoustic wave device according to claim 19, wherein thepiezoelectric film contains a divalent element and a tetravalent elementas the element.
 23. The acoustic wave device according to claim 22,wherein the divalent element is at least one of calcium, magnesium,strontium, and zinc, and the tetravalent element is at least one oftitanium, zirconium, and hafnium.
 24. The acoustic wave device accordingto claim 19, wherein the piezoelectric film contains a trivalent elementas the element.
 25. The acoustic wave device according to claim 24,wherein the trivalent element is at least one of yttrium and scandium.26. The acoustic wave device according to claim 19, wherein thepiezoelectric film has a crystal structure with a c-axis orientation.27. The acoustic wave device according to claim 19, further comprising atemperature compensation film having a temperature coefficient of anelastic constant opposite in sign to a temperature coefficient of anelastic constant of the piezoelectric film.
 28. The acoustic wave deviceaccording to claim 27, wherein the temperature compensation filmcontacts the piezoelectric film.
 29. The acoustic wave device accordingto claim 27, wherein the temperature compensation film contains mainlysilicon oxide.
 30. The acoustic wave device according to claim 19,wherein the electrode includes an upper electrode and a lower electrodefacing each other across the piezoelectric film.
 31. The acoustic wavedevice according to claim 30, further comprising: a temperaturecompensation film having a temperature coefficient of an elasticconstant opposite in sign to a temperature coefficient of an elasticconstant of the piezoelectric film; and conductive films that arelocated on a top surface and a bottom surface of the temperaturecompensation film and electrically connected to each other.
 32. Theacoustic wave device according to claim 19, further comprising: a firstpiezoelectric thin film resonator and a second piezoelectric thin filmresonator, each including the piezoelectric film and the electrode thatincludes upper electrode and a lower electrode facing each other acrossthe piezoelectric film, wherein the first piezoelectric thin filmresonator and the second piezoelectric thin film resonator are stacked,and a decoupler film is located between the upper electrode included inthe first piezoelectric thin film resonator and the lower electrodeincluded in the second piezoelectric thin film resonator.
 33. Theacoustic wave device according to claim 19, wherein the electrode is acomb-shaped electrode located on the piezoelectric film.
 34. Theacoustic wave device according to claim 33, wherein the acoustic waveexcited by the electrode when the element is added to the aluminumnitride constituting the film which is then charged with electricity isa surface acoustic wave or a Lamb wave.
 35. An acoustic wave devicecomprising: a piezoelectric film is an aluminum nitride film containinga divalent element and a tetravalent element, and electrodes connectedto the piezoelectric film to excite an acoustic wave propagating throughthe piezoelectric film, wherein a total of concentrations of thedivalent element and the tetravalent element is greater than or equal to3 atomic % and less than or equal to 35 atomic % when a total number ofatoms of the divalent element, the tetravalent element, and aluminum inthe aluminum nitride film defines 100 atomic %.
 36. An acoustic wavedevice comprising: a piezoelectric film is an aluminum nitride filmcontaining a divalent element and a tetravalent element, and electrodesconnected to the piezoelectric film to excite an acoustic wavepropagating through the piezoelectric film, wherein a ratio of aconcentration of the tetravalent element to a total of concentrations ofthe divalent element and the tetravalent element is greater than orequal to 0.35 and less than or equal to 0.75 when a total number ofatoms of the divalent element, the tetravalent element, and aluminum inthe aluminum nitride film defines 100 atomic %.
 37. An acoustic wavedevice comprising: a piezoelectric film made of an aluminum nitride filmcontaining a divalent element and one of a tetravalent or pentavalentelement; and electrodes connected to the piezoelectric film to excite,by the inverse piezoelectric effect, or cause, by a strain due to thepiezoelectric effect, an acoustic wave propagating through thepiezoelectric film, wherein the divalent element and one of thetetravalent or pentavalent element are substituted for aluminum atoms ofthe aluminum nitride film.
 38. The acoustic wave device according toclaim 37, wherein the piezoelectric film is the aluminum nitride filmcontaining the divalent element and the tetravalent element, andcontains at least one of titanium, zirconium, and hafnium as thetetravalent element.
 39. The acoustic wave device according to claim 38,wherein the piezoelectric film contains at least one of calcium,magnesium, strontium, and zinc as the divalent element.
 40. The acousticwave device according to claim 37, wherein the piezoelectric film is thealuminum nitride film containing the divalent element and thepentavalent element, and contains at least one of tantalum, niobium, andvanadium as the pentavalent element.
 41. The acoustic wave deviceaccording to claim 40, wherein the piezoelectric film contains at leastone of magnesium and zinc as the divalent element.